Focus target is not the same as lock on. Focus target behave the same as if you have a mob selected, and then use up/down arrows to highlight a party member, then cast a cure on them. After the cure it will revert to your focus target. It is a very nice feature. Focus target boss, switch around mobs or players. As soon as focus is lost on your current target it reverts to the boss.

Target lock on the other hand is an evil abomination that locks your camera and movement to the target and is mapped to the worst possible place on the controller guaranteeing you will hit it when you are trying to move out of an AOE rapidly.